Biotin possesses an extraordinarily strong binding affinity to avidin and streptavidin (Kd≈10−15M).(4) The quaternary structure of avidin (Mw≈66 kDa) consists of a non-covalent tetramer which combine into the active form.(5, 6) Therefore each avidin unit is capable of binding four biotin molecules.(6, 7) This binding property can be utilized in the development of a universal tracer that could be used, for example, in the management of various tumor types (8) based on pretargeting concept as well as in organ transplantation medicine.
In vivo imaging using pre-targeting is a known technique. Thus, biotin-DOTA conjugates [where DOTA is the macrocyclic chelator 2,2′,2″,2′″-(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid] are described in U.S. Pat. No. 5,608,060. The DOTA-biotin conjugates of U.S. Pat. No. 5,608,060 are as follows:
                where L is a linker group incorporating a phenylene-CH2— moiety Axworthy et al [Proc. Nat. Acad, Sci. USA, 97(4), 1802-1807 (2000)] disclose pretargeting using avidin-conjugated antibodies and the 90Y-complex of a DOTA-biotin conjugate for radioimmunotherapy in vivo. The DOTA-biotin conjugate used was that described in U.S. Pat. No. 5,608,060.        
WO 02/066075 discloses conjugates of biotin with chelating agents based on functionalisation of biotin with diamines H2N-Q-NH2, where Q is —(CH2)n— and n is 4 to 12. The chelator of WO 02/066075 is an N4 macrocycle, of variable ring size, which may be attached at several locations. WO 02/066075 also discloses radiometal complexes of the conjugates, with 25Fe, 52mMn, 55Co, 64Cu, 67Ga, 68Ga, 99mTc, 111In, 123I, 125I, 131I, 32P, 47Sc, 67Cu, 90Y, 109Pd, 111Ag, 149Pm, 186Re, 188Re, 211At, 212Bi, 213Bi, 105Rh, 153Sm, 177Lu and 198Au. The only specific complex exemplified by WO 02/066075 is with 90Y.
Lewis et al [Nucl. Med. Biol., 29, 701-706 (2002)] disclose the preparation of a 66Ga complex of the DOTA-biotin conjugate of U.S. Pat. No. 5,608,060. The main focus of Lewis et al was on the feasibility of obtaining 66Ga in sufficient purity for radiopharmaceutical applications, rather than applications of the labelled DOTA conjugate.
Hainsworth et al [Bioconj. Chem., 16(6), 1468-1474 (2005)] disclose DOTA-biotin conjugates having amide or amine linkages in the linker group between the DOTA chelator and the biotin.
Forster et al [J. Nucl. Med., 47(1), 140-149 (2006)] investigate methods of reducing the renal radiation dose in radioimmunotherapy. They used DOTA-biotin conjugates labelled with 67Ga as a model system. The DOTA-biotin conjugate used was that of Axworthy et al (above), i.e. that of U.S. Pat. No. 5,608,060.
Blom et al [Bioconj. Chem., 20(6), 1146-1151 (2009)] disclose the 68Ga-labelling of DOTA-biotin conjugates where the effect of the linker group between the DOTA and the biotin is studied.
There is still a need for alternative and/or improved radiotracers and radiopharmaceuticals suitable for pre-targeting imaging using the biotin/avidin system in vivo.